The present invention relates to a process for the removal of thiosulfate ion (S.sub.2 O.sub.3.sup.2-) from Stretford solution. More particularly, the process of the present invention utilizes hydrogen peroxide or other peroxygen compounds at acidic pH for reaction with the thiosulfate ion in the Stretford solution.
The so-called "Stretford Process" removes hydrogen sulfide from natural and process gases by oxidizing the sulfide ion to elemental sulfur with air, vanadium (V) salts and anthraquinone disulfonates.
Stretford solution is always kept on the alkaline side. In the alkaline range, the solution absorbs the sour gas or tail gas (hydrogen sulfide) and converts it to sulfur by the use of the catalyst.
The term "Stretford solution" is used to define a catalytic solution containing alkali vanadates and anthraquinone catalysts. Petrochemical facilities and natural gas processing facilities which process large amounts of hydrogen sulfide containing gasses pass these gasses into the Stretford system to react with air and cause the sulfide to be oxidized to sulfur. This sulfur is then separated from the Stretford solution and sold for use in a variety of chemical processes.
During this process a portion of the sulfide is instead converted to thiosulfate and sulfate. This occurs, according the report by Trofee, T. W., et al., Stretford Process Operations and Chemistry Report: Final Report, prepared by Radian Corporation, Austin, Tex. for the Gas Research Institute GRI93/0129, Ill.; November, 1993; pages 2-to 2-16 by the reaction of oxygen with polysulfide (S.sub.n.sup.2-) at alkaline pH, to form thiosulfate, (S.sub.2 O.sub.3.sup.2-) and, also by the reaction of sulfur, at alkaline pH. Sulfate also is a by-product of the oxidation of the sulfide to sulfur.
Thiosulfate accumulates in the solution to a higher concentration than sulfate. Different facilities determine the maximum tolerance for thiosulfate in their Stretford reactor in different ways, such as increased chemical consumption and costs due to the need to remove a portion of the contaminated catalyst solution and replacing it with fresh catalyst solution (blowdown), high blowdown disposal costs, poor solubility of the catalysts in the contaminated Stretford solution when a high level of thiosulfate is present, poor removal of sulfide from the gas handling stream due to poor catalyst performance, and also higher corrosivity of the solution on the tanks and working parts of the reactor. In any case, when the concentration of thiosulfate reaches between 150 to 400 g/l as sodium thiosulfate pentahydrate, the solution is spent and needs to be replaced.
In the past this outcome resulted in the necessity of disposing of the spent solution. In some cases it is possible to simply dump this solution into a landfill by absorbing it onto fly ash and sealing in steel drums. This is a less viable option in recent years than it was in the past because of increasing environmental regulations. Disposing of this material in this way is a burden to the landfill, the community and the source of the product for many years to come. The source, or party generating the waste, is forever responsible for the environmental harm that may come from that material. In fact, in California, this material is classified as a hazardous waste.
It is possible to ship the spent solution to an off-site facility for chemical processing to oxidize the thiosulfate and recover the vanadium for reprocessing. This is becoming more difficult as well due to the reluctance of facilities to accept this material.
Both of the above disposal options result in loss, at least in part, of the valuable catalyst components that are present in the Stretford solution, meaning that fresh solution must be made up to continue the process. This represents additional reagent and manpower costs for the operating facility.
Another option is to desalt the spent Stretford solution using a commercial process that removes the thiosulfate and sulfate as elemental sulfur and Glauber's salt (Na.sub.2 SO.sub.4. 10H.sub.2 O). This process is at least as expensive as disposing of Stretford solution by the above methods costing about $0.78-$0.85 per gallon.
It is therefore an object of the present invention to find a way of treating the spent Stretford solution to remove the thiosulfate therefrom in an efficient and technically superior manner.
Another object of the invention is to avoid disposal problems that have arisen with prior methods of treating spent Stretford solutions.
In the past, there have been studies using hydrogen peroxide to oxidize thiosulfate in petrochemical and mining wastes, but none have used the reaction conditions of this invention and none have produced the large amounts of marketable sulfur that this process produces. In both cases, the products described in the reaction are sulfate and polythionate or polysulfide rather than sulfur.
In an article, Redox Chemistry of H.sub.2 S Oxidation by the British Gas Stretford Process Part IV: V--S--H.sub.2 O Thermodynamics and Aqueous Vanadium (V) Reduction in Alkaline Solutions, by Kelsall, Thompson and Francis, Journal of Applied Electrochemistry, 23 (1993), 417-426, the authors even go so far as to describe the reaction of hydrogen peroxide with various vanadate species but do not describe the effect of adding hydrogen peroxide under the conditions used in this invention.